This invention is directed to a closed steam thermodynamic power cycle and particularly the employment of a screw expander in such a cycle wherein the expansion of the steam occurs between rotor lobes, and particularly one in which heat exchange is accomplished during expansion to improve cycle efficiency.
Lysholm built an early prototype of the rotory screw compressor in 1934. Some of his development work was described in the Proceedings of the Institution of Mechanical Engineers, Vol. 150, No. 1, pages 11-16 and four plates, 1943. The Lysholm-type rotary screw compressor has two rotors with intermeshing lobes. Within the intermesh of the lobes and housing, compression or expansion takes place depending upon the direction of rotation and the consequent direction of flow. Two helical rotors comprise the working parts of this screw expander. The male rotor generally has four lobes and rotates 50% faster than the female rotor which has six flutes between which are grooves in which the lobes engage. Other ratios of lobes to flutes are also used. Gas is expanded in the spaces between the housing, the lobes and the grooves. The lobes and the grooves are helical so that the space appears to move progressively toward the outlet end of the housing and the space becomes progressively larger along the length of the rotors as the rotors rotate. Thus, gas taken in at the inlet port at the high pressure end is expanded in the spaces as the rotors turn and it is finally delivered at lower pressure from the outlet port at the delivery end of the housing. The inlet and outlet ports are automatically covered and uncovered by the shaped ends of the rotors as they turn. Most of the work on the Lysholm type of screw machine has been in the area where the machine is used as a compressor. Most of the patents are owned by Svenska Rotor Maskiner, which devoted the pioneer effort in this art and appears to hold most of the patents. The company is located in Nacka, Sweden.
Nilsson U.S. Pat. No. 3,245,612 and Schibbye U.S. Pat. Nos. 3,283,996 and 3,423,017 are particularly directed to the shapes of the lands and the grooves on the rotors, but show the porting in general organization. These patents show how compression and expansion are achieved in such a structure. Sprankle U.S. Pat. No. 3,977,818 shows a particular helical screw expander having a throttle valve particularly useful for the expansion of geothermal fluid streams.
Furthermore, Sprankle U.S. Pat. No. 3,751,673 shows a helical screw expander in a geothermal power utilization system wherein saturated liquid which is the direct product of the geothermal well is expanded and partially flashed in the screw expander during the expansion process. The problem with that cycle is that there is very little enthalpy change through the expander so that it is difficult to extract adequate power to warrant such an installation.
A publication which is related to the same art, the direct expansion of a geothermal fluid stream through a screw expander, is "The Helical Screw Expander Evaluation Project" by Richard A. McKay of the Jet Propulsion Laboratory, published May, 1977 in the Proceedings of the Twelfth Intersociety Energy Conversion Engineering Conference at pages 899-903.
When the screw expander is used as an expansion engine in an external combustion circuit, then the screw expander needs cooling. The present materials are not sufficiently well developed to permit operation of the screw expander at local temperatures without extracting some heat. Lindhagen U.S. Pat. No. 2,808,813 discusses the use of the screw expander in such an environment. Nilsson U.S. Pat. No. 2,755,990 describes a housing by which the screw expander can be cooled by cooling air circulating in a chamber surrounding the structural part of the housing. Screw machinery of this type has found its greatest use as gas compressors, and the cooling is in the nature of removing the heat of compression. The prior art includes no effort to improve cycle efficiency when the screw machine is employed as an expander for extracting work from a thermal fluid stream.